Voltage regulators are important components of consumer electronic devices and other electronic devices. A voltage regulator provides a nearly constant voltage output level at a particular connection. For example, a nearly constant voltage may be provided to a backlight of a liquid crystal display (LCD) of an electronic device. In another example, a nearly constant voltage may be provided to an output node to detect the presence, or not, of an attached device. Multiple voltage regulators may be present in electronic devices. In some configurations, multiple voltage regulators are coupled to the same output node and operated in tandem to provide different voltage levels at that output node.
A voltage regulator may be capable of producing multiple levels of a nearly constant voltage output. However, the range of levels available from a voltage regulator may be limited. Further, different voltage regulators may be more or less efficient in different ranges of voltage output levels. Thus, for example, two voltage regulators may be coupled to an output node when a desired output voltage range for the node is 0-3 Volts. A first voltage regulator may provide the output voltage for a low portion of the 0-3 Volt range, and a second voltage regulator may provide the output voltage for a high portion of the 0-3 Volt range. An example of this arrangement is shown in FIG. 1.
FIG. 1 is a block diagram illustrating multiple voltage regulators coupled to an output node according to the prior art. A first voltage regulator 102 may be coupled to an output node 106, and a second voltage regulator 104 may also be coupled to the output node 106. The first voltage regulator 102 may be configurable to provide output levels of 1.86, 2.0, and 2.3 Volts. The second voltage regulator 104 may provide an output level of 2.75 Volts. When a low output voltage is desired at the output node 106, the first regulator 102 may be active and driving the output node 106 to the desired low output voltage. When a high output voltage is desired at the output node 106, the second regulator 104 may be active and driving the output node 106. Thus, the driver of the output node 106 may switch back and forth between the first regulator 102 and the second regulator 104. When one of the regulators 102 and 104 switches on to become the driver for the output node 106, the regulator experiences a current step at its output. That is, when one of the regulators 102 and 104 is off then it is not outputting any current. However, when the regulator 102 or 104 switches on, then it must immediately begin providing current at a level required by the device connected to the output node 106. The abrupt increase in current output by the regulator 102 or 104, referred to as a current step, may result in undesirable behavior at the output node 106.
FIG. 2 is a graph illustrating a result of a current step on the nearly constant voltage output at an output node according to the prior art. A line 202 illustrates a voltage output at the output node 106 of FIG. 1. Lines 204 and 206 illustrate nearly constant voltages V2 and V1 generated by the regulators 104 and 102, respectively. Prior to time 212, the second regulator 104 is driving the output node 106 at voltage V2 of line 204. At time 212, the first voltage regulator 102 takes over driving the output node 106. The current step at the output of the first voltage regulator 102 causes a droop 222 in the output voltage of line 202 during time period 216. After a transition time period 216, the voltage of line 202 eventually stabilizes at voltage V1 of line 206. The transition time period 216 can be a relatively long period during which the droop 222 may cause errors in the electronic device. One source of the droop 222 is the limited bandwidth of an amplifier within the voltage regulator 102 or 104. Another source of the droop 222 is a slow transition time for the gate voltage of pass transistors 102B and 104B coupled to amplifiers 102A and 102B, respectively, of the regulators 102 and 104. This slow transition is shown in line 208 of FIG. 2 showing the gate voltage stabilizing during the transition time period 216.
One example of an error may be illustrated with a headset for mobile device, such as a cellular phone or a media player. A VMICBIAS voltage may be supplied to a third terminal of a headphone jack of the mobile device (where the first and second terminals provide audio to the headphones). This bias voltage allows a microphone in line with the headphones to record sounds from the environment, such as a person speaking over the telephone. Additionally, a measurement of the voltage may be used to determine whether a headset is connected to the headphone jack. The droop 222 of VMICBIAS voltage 202 shown in FIG. 2 may cause erroneous operation of the microphone or may cause erroneous detection of the presence or absence of a headset. These errors may affect operation of the mobile device. For example, erroneous microphone operation may cause speech during a telephone call to be corrupted. In another example, erroneous headset detection may cause the mobile device to incorrectly turn on or turn off speakerphone operation of the mobile device.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved voltage regulators, particularly for audio devices and other consumer-level devices. Embodiments described here address certain shortcomings but not necessarily each and every one described here or known in the art.